1. Field of the Invention
The present invention relates generally to a method of treating industrial waste and hazardous waste, and more particularly to a method of removing heavy metals from incinerator fly ashes.
2. Description of the Related Art
15-25% ashes are derived from the waste incinerator and the ashes are divided into bottom ash and fly ash. Bottom ash contains ceramics, glass, iron, aluminum, pure ash etc., and usually meets the criteria of Toxicity Characteristics Leaching Procedure (TCLP), thus normally classified as a nonhazardous solid waste. On the contrary, fly ash contains heavy metals, such as Cd and Pb, which usually fail in TCLP test and ordinarily classified as a hazardous waste. According to the relative laws, fly ash cannot be mixed with bottom ash and must be treated prior to disposal. The conventional treatments of hazardous fly ash include solidification, stabilization, encapsulation and thermal treatment. Besides, some new methods including synthetic zeolite treatment and fluidizing electrolysis treatment for incinerator fly ash are developing. Conventional treatments for fly ash are described in brief as follows:
1. Solidification treatment: Hazardous waste is mixed with cement, pozzolanic material or other chemical fixing agent to solidify the waste, and to prevent the hazardous materials of the waste from leaching. A common fixing agent used in the solidification of incinerator fly ash is cement. Cement solidification treatment is easily operated, but it generates larger size of solidified material which will not be able to recycle and need to be monitored in an independent landfill site.
2. Stabilization treatment: Hazardous waste is added with chemical stabling agent for reaction and transformed to be a stable and nonpoisonous material. The conventional stabling agents include epoxy, urea formaldehyde, polyurethane, polyester, asphalt and other chemical compounds.
3. Encapsulation treatment: Hazardous waste is encased into a suitable case as in a capsule for isolation. Since the cost of this treatment is high, so encapsulation is not suitable for fly ash but for special wastes, like nuclear waste.
4. Thermal treatment: Inorganic hazardous waste is heated to a temperature over 1,200° C. and forms a ceramic-like or glass-like material. The cost of thermal treatment of incinerator fly ash is high comparing with those of other treatments. Recently, some researchers are looking for a possibility of mixing incinerator fly ash with waste irons in an electric arc furnace (EAF), but it is found that heavy metal in fly ash is more accumulated.
This patent inventor had done many research for the removal of heavy metals in incinerator fly ashes. According to the generation place, fly ashes are classified as “original fly ashes (OFA)” and “reacted fly ash (RFA).” OFA was generated at boiler, cyclone, and economizer. RFA was generated at semi-scrubber and baghouse. The patent inventor's research is described in brief as follows:
1. Acid flushing: Two MSW incinerators fly ashes were packed into column and flushed by hydrochloric acid (pH=2). The flushed OFA was then tested by TCLP and observed that the removals of Cd, Pb and Zn were 82%, 40% and 56% respectively, but all couldn't met the TCLP standard. Nevertheless, the removal of Pb in RFA was 89% that passed the TCLP test.
2. Ultrasonic extraction: Two incinerator fly ashes were sampled and treated by ultrasound. The removals of Cd, Pb and Zn in OFA were 31%, 25% and 11% respectively, but all exceeded the TCLP criteria. The removal of Pd in RAF was 89% that passed the TCLP criteria.
3. Electrokinetic treatment: Fly ash has high alkalinity so that much power is needed in the electrokinetic treatment. According to a constant 50 V, 7 days electrokinetic treatment for incinerator fly ash, electricity consumption was estimated about 5,700-7,000 kWh/ton. It means that electrokinetic treatment is too expensive to be used.
4. Acid extraction: MSW incinerator fly ashes were extracted by acid solutions which including hydrochloric acid, nitric acid, sulfuric acid, citric acid and electrolytic acid. The concentrations of acid solutions were 0.1, 0.01, and 0.005 M. After a twenty-four hour shaking extraction, the removals of Cd and Zn were in a range of 60-90% and the removal of Pb was 7-71%. It was found that increasing the concentration of acid solution will increase the removal of Cd and Zn except Pb.
5. Non-acid extraction: MSW incinerator fly ashes were extracted by EDTA, SDS and distilled water (DW). The concentrations of non-acid agents are 0.1, 0.01, and 0.005 M. The results suggested that the removals of Cd and Zn were in a range of 88-100%, and Pb was 2-77%. The higher concentration of non-acid solution has the greater removal of heavy metal, except the 0.1 M EDTA sample.
6. Sequential extraction (DW, sulfuric acid, ultrasound): MSW fly ashes were first extracted by distill water, and then extracted by sulfuric acid. The results shown that: (1) OFA: The removal of Cd in TCLP was 6-30%, but total content of Cd wasn't change significantly. On the contrary, the removal of total content of Zn was 39-47%, but the concentration of Zn in TCLP increased. (2) RFA: The removal of Pb in TCLP was 58-70%, and was 26-30% in total content. Such sequential extraction samples were further treated by an ultrasound-assisted extraction for ten minutes. The results indicated that: (1) OFA: The removal of Cd didn't change much in both of TCLP and total content, and the concentration of Zn in TCLP also increased. (2) RFA: The removals of Pb in TLCP and total content were 59-74% and 25-39% respectively, which hinted that ultrasound could improve the removal performance.
7. Sequential extraction (DW, sulfuric acid, centrifugation): MSW fly ashes were sequential extracted by DW and sulfuric acid, and then the samples were centrifugal separated in 2,500, 5,000 and 10,000 rpm for 5 and 10 minutes. It was found that centrifugation benefits the removal of heavy metals from fly ashes.
8. Sequential extraction (DW, microwave): MSW fly ashes were continuously extracted by DW for three times to release electrolytes. The results implied that: (1) OFA: Cd in TCLP was removed 31-78%, which met the TCLP criteria. Besides, Cd and Zn in total content were removed 22-29% and 43-51% respectively, but the removal of Zn in TCLP was unstable. (2) RFA: The removals of Pb in TCLP and total content were 53-58% and 19-26% respectively. Samples were further treated by a 2,450 MHz/500 W microwave oven for 5-30 minutes. The results shown that: (1) OFA: The removal of Cd in TCLP was 26-60%, and Zn in TCLP was worse than before. The removals of Cd and Zn in total content were 6-36% and 39-59% respectively. (2) RFA: The removal of Pb in TCLP was 65-80%; in total content was 34-67%. It indicated that microwave-assisted treatment could increase the metal removal.
9. Extraction by SDS, Fe(NO3)3 and artificial Fe(NO3)3: MSW incinerator fly ashes were extracted by a critical micelle concentration (CMC) of SDS, Fe(NO3)3 and artificial Fe(NO3)3. The concentrations of Fe(NO3)3 and artificial Fe(NO3)3 were 0.1, 0.01 and 0.005 M. According to the TCLP test, Cd and Pb were decreased but they still exceeded the TCLP criteria. General extraction performance was shown that: SDS>artificial Fe(NO3)3≧Fe(NO3)3, and Fe3+ and the agent concentration didn't affect obviously on the removal (or displacement) of heavy metals.
10. Sequential extraction by ultrasound: MSW incinerator fly ashes were mixed with DW and settled for a day, and then treated by an ultrasound-assisted extraction for five minutes. After seven cycles, the results suggested that: (1) OFA: The removals of Cd in TCLP and Zn in total content were observed, but Zn was fail in TCLP. (2) RFA: The concentrations of Pb passed TCLP criteria in the first and second cycles.
In conclusion, the inventor had tried many methods for removing heavy metals from MSW incinerator fly ashes. Such methods can be classified as: (1) flushing, (2) ultrasound-assisted extraction, (3) electrokinetic treatment, (4) shaking extraction by acids (hydrochloric acid, nitric acid, sulfuric acid, citric acid, and electrolytic acid) and non-acid solutions (EDTA, SDS, DW, Fe(NO3)3, and artificial Fe(NO3)3), (5) centrifugal separation, and (6) microwave-assisted extraction. Even each of those methods had got a certain performance in one or two metal removal, but none of them could pass all metals in TCLP criteria.